A simple touch on the valve requires almost no tehnology to get the proper temperature. Substituting a voice control on the valve requires at minimum some sort of sound sensitive system and an actuating motor which adds greatly to the problems of adjusting and even with voice control to replace your hand there will be searching to get the right adjustment. Interposing massive technology introduces huge expense, great capability for error, dangerous mix of electricity with plumbing and no better control than a simple twist of a valve.

I think it is a good idea. We should be at least able to stop and start the flow of water with voice command. We should also be able to control the amount of water that is used by toilets by selecting a low/medium/high button.

4-20mA Proportioning Valves
Choose a valve, buy a PID controller for your computer, and tie voice recognition into the controller using LabView software, a National Instruments product.
All of the Home Automation gizmos are completely doable, but there still doesn't seem to be much of a market for this type of gear.

You would hook your standard valve set in parallel with the automated valves. That way if the automated system failed, you could still take a shower.